The present invention relates generally to heads for use in a magnetic storage disk drive, and more particularly to a method and apparatus for detecting and controlling head fly height and signal PW50.
Conventional magnetic storage devices include a magnetic transducer, or xe2x80x9cheadxe2x80x9d suspended in close proximity to a recording medium, for example a magnetic disk having a plurality of concentric tracks. The transducer is supported by an air bearing slider mounted to a flexible suspension. The suspension, in turn, is attached to a positioning actuator. During normal operation, relative motion is provided between the head and the recording medium as the actuator dynamically positions the head over the desired track. The relative movement provides an air flow along the surface of the slider facing the medium, creating a lifting force. The lifting force is counter balanced by a pre-determined suspension load so that the slider is supported on a cushion of air. Air flow enters the xe2x80x9cleading endxe2x80x9d of the slider and exits from the xe2x80x9ctrailing endxe2x80x9d. The head resides at the trailing end which tends to dip closer to the recording surface than the leading end.
As discussed above, the sliders ride on a cushion of air generated by the rotation of the magnetic disk. The upward force on the slider provided by this air cushion is directly proportional to the rotational velocity and as a consequence varies from the inner disk diameter (ID) to the outer disk diameter (OD). Since the sliders tend to be sensitive to changes in the upward force, the fly height will vary across the disk. Surface defects will also cause the head to xe2x80x9cfly highxe2x80x9d. This is the most important error condition.
The recording medium includes information encoded in the form of magnetic transitions. The information capacity or areal density of the medium is determined by the transducer""s ability to sense and write distinguishable transitions. An important factor affecting areal density is the distance between the head and the recording surface referred to as xe2x80x9cfly heightxe2x80x9d. It is desirable to fly the transducer very close to the media to enhance transition detection without permitting transducer contact with the medium surface. Some fly height stability is achieved with proper suspension loading and by shaping the air bearing slider surface (ABS) for desirable servo dynamic characteristics.
Another important factor affecting fly height is the slider""s resistance to changing conditions. The air bearing slider is subject to a variety of changing external conditions during normal operation. Changing conditions affecting fly height include for example, change in relative air speed and direction and variation in temperature. If the transducer fly height does not stay constant during changing conditions, the data transfer between the transducer and the recording medium may be adversely affected.
Fly height is further affected by physical characteristics of the slider such as the shape of the ABS. Careful rail shaping, for example, will provide some resistance to changes in air flow. Another physical characteristic often found in conventional sliders is curvature along the length of the ABS from leading end to the trailing end. This curvature is referred to as xe2x80x9ccrownxe2x80x9d and may be either concave or convex with respect to the ABS. Crown variation is generally one of two types namely, process variation or temperature variation.
However, it has been determined that it may be difficult to anticipate all of the factors which affect xe2x80x9cfly heightxe2x80x9d, and consequently, some variations in the fly height result randomly. These variations should be identified when they occur because if the fly height is too large, the result is that information is not being recorded properly on the media. This condition may make it necessary to return the write head to the same location and rewrite the information.
FIG. 1 illustrates a circuit of the prior art. In this circuit, two filters are connected in parallel. These filters maybe a 10 pole band pass filter. The filter 100 may have a band pass frequency of F while the filter 102 has a band pass frequency of frequency 3F. The filter 100 filters out everything but the first harmonic of the input signal, and the filter 102 filters out everything but the third harmonic of the input signal. The output of filter 100 is connected to a peak detector circuit 104 to detect the peaks from the output of the filter 100. Additionally, a peak detector circuit 106 is connected to the output of filter 102. The peak detector circuit 106 detects the peak signal from the filter 102. The output of the peak detector circuit 104 and the output from the peak detector circuit 106 are connected to a dividing circuit 108 to divide the outputs from peak detectors 104 and 106. The problem with this circuit is that it is complex and difficult to implement. A circuit is required that provides accurate indication of fly height and is easy to implement.
The present invention determines fly height by sensing changes in the PW50 of a pulse being output from the head of a disk drive system. PW50 is essentially an indication of the sharpness of the head output signal. The present invention senses changes in the degree of sharpness of this signal as an indication of the height of the head flying over the surface of the disk drive. The signal sharpness (or PW50) of the signal is determined from the ratio of the amplitude of the peak signal and the area under the curve of the signal. This ratio is an indication of fly height. The present invention eliminates the need to employ band pass filters to filter the first harmonic and the third harmonic. The present invention determines the PW50, which is a measure of the pulse width at 50% of the peak amplitude, from the input signal. The present invention uses a peak detector in parallel with a full wave rectifier and integration circuit to determine the ratio which is an indication of the head fly height.